Diesel powered engines are typically ignited by a mixture of injected fuel and hot compressed air. While external ignition devices (e.g., spark plugs) are not required, cold weather conditions pose substantial difficulties with respect to starting diesel engines. That is, as ambient temperatures decrease, the temperature of the internal combustion chamber may not be high enough to effectuate proper ignition of the injected fuel. Accordingly, various starting aids are commonly incorporated within diesel powered vehicles in order generate a sufficient amount of heat to initiate ignition.
One preferred method for facilitating ignition within diesel engines involves the employment of glow plugs. Generally, glow plugs are disposed within each combustion chamber of the diesel engine, and in operation, they effectively heat the combustion chamber.
An additional issue attributed to diesel engines running at low ambient temperature conditions concerns undesirable exhaust emissions, such as “white smoke.” White smoke can be attributed to the release of unburned hydrocarbons as a result of misfire or incomplete combustion within the engine. Low temperature ambient air and low engine loads operate to reduce the temperature of the charge mixture within the engine to a degree that the combustion event becomes unstable.
Diesel engines must also be designed to combat unburned soot particles resulting from incomplete combustion. As such, most diesel engines feature a diesel particulate filter (DPF), i.e., a device designed to collect the particulate matter. However, as the quantity of particulate matter increases in the DPF, the DPF must be cleaned to restore its operability. The process of removing accumulated particulate matter from the DPF is most commonly referred to as regeneration. Regeneration involves the process of increasing the temperature of the DPF to about 550 degrees Celsius, such that the particulate matter combusts or burns. Typically, however, such a temperature is only achieved during high load and high engine speed conditions. As a result, when diesel engines operate at low loads, fuel may be injected in small amounts, and the temperature of the exhaust gas falls below regeneration levels.
Manufacturers have proposed various technologies for elevating exhaust gas temperatures to a level suitable for carrying out regeneration. For instance, it is known in the art to include catalysts for oxidizing the unburned components in the exhaust gas in order to elevate the exhaust gas temperature. Additionally, the exhaust gas temperature can be increased by a process known as post-injection. Post-injection refers to the method of providing additional fuel in the combustion chamber or exhaust gas system. The additional fuel does not burn in the engine cylinder, but rather, oxidizes with catalysts disposed in the exhaust pipe in order to increase exhaust gas temperature. Nevertheless, following a cold start, the engine may have to operate for over 100 seconds before a sufficient amount of heat energy is generated in order to initiate catalytic activity. To reduce this time, heating devices may be provided to introduce additional heat in the exhaust path.